The invention relates to a hydraulic circuit suitable for motor vehicles and particularly hydraulic circuits having a camshaft adjuster, and also to corresponding control elements.
In the hydraulic circuits of motor vehicles, hydraulic pistons are used to vary the position of a connected mechanical element, such as a camshaft for example. One type of hydraulic piston may be a swivel-motor-type rotary piston or even a radial piston, also known as a hydraulic motor, which is capable of varying its position in a gyratory manner within a certain angular range.
The piston moves within a housing, the piston forming, on both sides, hydraulic spaces which are varied in an oppositely oriented manner. This means that, if one hydraulic chamber grows as a result of a variation in the position of the hydraulic piston, the corresponding chamber located opposite the piston is reduced in corresponding measure, and vice versa. As is known, the hydraulic chambers are configured in the same manner, so that the growth of one hydraulic chamber, volume-wise, contributes to the same reduction, volume-wise, in the other, corresponding chamber. In this case, the variations in volume are equivalent or even identical in terms of amount.
One very important hydraulic circuit of a motor vehicle is the camshaft-adjuster circuit which starts in the engine sump and which adjusts, via suitable valves and a swivel-motor-type camshaft adjuster, the relative location of the camshaft in relation to a driving shaft, such as the crankshaft or another camshaft for example. The adjustments take place in the direction of an earlier or later point in time with respect to the angle of rotation of the driving shaft or with respect to the position of the piston. In contrast to, for example, closed systems which have a single hydraulic circuit, in just the same way in which known motor-vehicle transmissions are constructed, a system of this kind is regarded as an open system which operates with variable volumes of oil, because a number of hydraulic circuits are present in the internal-combustion engine, starting within the engine sump.
Other known hydraulic circuits in the motor vehicle may include, for example, transmission control systems which are attached, either to the central hydraulic circuit, which is supplied with engine oil, or to an independent, self-contained hydraulic circuit.
Particularly in the case of multiple hydraulic loadings resulting from a strung-on hydraulic system, motor vehicle manufacturers are calling for the smallest possible loading on the hydraulic pump, which has to supply all the consumers. This lowers the parasitic loadings on the internal-combustion engine, and this, in turn, contributes to increasing the efficiency.
Numerous ways in which the over-supplying of the hydraulic consumers can be reduced can be inferred from US 2005/0072397 A1, which primarily addresses the delivery quantities of the hydraulic circuit. According to one aspect of the invention described therein, rotational-speed-dependent delivery quantities from an oil pump which is mechanically coupled directly to the internal-combustion engine are reduced by additional delivery or storage apparatuses.
Another important call from internal-combustion engine manufacturers is the desire to be able to incorporate the quickest possible camshaft adjusters in the internal-combustion engine. As a rule, the speed of adjustment of the camshaft adjusters is increased by correspondingly high oil delivery quantities. Many motor vehicle manufacturers are calling for adjusters with speeds of adjustment of 100°/sec. Adjusters whose speed of adjustment is indicated by means of a single extreme value are often encountered in the literature. What is important, however, is the speed of adjustment over all the rotational speeds of the internal-combustion engine, which speed should be as constant or linear as possible. Thus, in some cases, speeds of adjustment of more than 200°/sec at certain points are described, which, on closer investigation, have a purely singular character with respect to the rotational speed. If these data are studied more closely, it can be established that they often relate to high rotational speeds with low oil temperatures. It is true that a quick camshaft adjuster is obtained by incorporating a larger oil pump, but the output or efficiency of the internal-combustion engine goes down.
From published specification EP 0 388 244 A1, a system is known which, in a completely enclosed manner and with two anti-parallel circuit arrangements, adjusts, via a valve, the relative location of a driven shaft in relation to a driving shaft by equalising, from one chamber to the second chamber, a volume of oil which is constant overall. The teaching of the printed specification, which is summarised, for example, in the main claim and in FIGS. 3 and 7, is to be viewed more as theoretical since, as is known, leakages occur in the hydraulic circuit of a camshaft adjuster.
In the technical literature, particularly in the article “A camshaft torque-actuated vane-style VCT phaser” by authors Frank Smith and Roger Simpson, reprinted as SAE Article 2005-01-0764, it is proposed, for example, that the pump of the hydraulic circuit be relieved of pressure through the fact that the pump continues to compensate for leakages from the adjuster only while a hydraulic compensating system which is normally closed is present between the two oppositely oriented chambers of the adjuster. The speeds of adjustment put forward in the charts lead to the assumption that the system put forward only operates with suitably large quantities of oil in the hydraulic circuit of the adjuster. In conventional engines in small motor vehicles, which are known, above all, in Western Europe and Japan, the system described would probably find few applications because engines of this kind are supposed to manage with markedly smaller filling quantities (often less than 5 liters of engine oil). A patent which belongs in the same category can be seen in U.S. Pat. No. 5,657,725.
Utilisation of the moment fed into the camshaft adjuster by the camshaft for the purpose of adjusting the camshaft adjuster into an early position is known from DE 101 58 530 A1 and DE 10 2005 023 056 A1. Whereas DE 101 58 530 A1 aims to use the technique in order to pass into the early position more swiftly when the engine drops from a hot-running phase into a lower rotational-speed range, DE 10 2005 023 056 A1 aims to ensure, above all in the event of a failure of the supply pump, that the camshaft is twisted into a position of the kind in which further operation in the early position is possible. For this purpose, DE 101 58 530 A1-uses a non-return valve with a pressure-equalising valve in the camshaft adjuster itself, whereas DE 10 2005 023 056 A1 proposes to arrange a number of non-return valves around the pump.
DE 602 07 308 T2 proposes using a valve or a changeover switch which differentiates between two states, namely between a high rotational-speed range in which an oil pressure-actuated camshaft adjustment takes place, and a low rotational-speed range in which a camshaft moment-actuated camshaft adjustment takes place. The changeover switch switches to and fro between the two states in dependence upon the operating conditions.
As can be seen, the prior art teaches the utilisation of camshaft moments for certain modes and types of operation. The hydraulic circuits have accordingly been designed for the tasks set.
In order to improve the speed of adjustment, it is known from DE 102 05 415 A1 or its American relative U.S. Pat. No. 6,941,912 B2, which are based on in-house developments by the Applicant, to interconnect a group of valves, in particular four valves that work with pistons, in order to clear a bypass line through which hydraulic medium can be transferred from one chamber to the other for the purpose of increasing the speed of adjustment. Apart from that, the system is an open one which is supplied from a delivery pump. From one of the exemplified embodiments, it can be seen that a bypass arrangement can be produced by means of a nested double-piston arrangement of the hydraulic shunt. According to this exemplified embodiment, the bypass arrangement is arranged, in a manner uncoupled from the shunt and independently, with a valve group which comprises a number of pistons and is set up in the rearward wall of the camshaft adjuster.
In the present invention, an approach has therefore been sought after for the purpose of designing a hydraulic system which offers a high, and also virtually constant, speed of adjustment of the hydraulic piston, as far as possible independently of the operating parameters; which at the same time offers a high regulating quality; which represents a low load for the oil pump of the internal-combustion engine; and which can be incorporated even in small-volume engines, e.g. 1.3 or 1.8-liter engines which have fewer gas exchange valve restoring springs than, for example, the V6 engine in the technical article described above.
In camshaft adjusters, the regulating quality is indicated, inter alia, in angular degrees within which the camshaft adjuster oscillates, although a defined, constant position according to the pressure loading from the supply pump is desirable. The deviation from the theoretically set position in angular degrees is then designated as the regulating quality.
The inventors also set themselves the object of being able to use the system to be designed, even in fully variable valve drives which are described in greater detail in, for example, patent applications WO 2004/088094, WO 2004/088099 and U.S. Pat. No. 6,814,036 A or EP 1 347 154 A2.